Fix following comma detection in code completion (#29503).

pkg/analysis_server/test/services/correction/flutter_util.dart

Index: pkg/analysis_server/test/services/correction/flutter_util.dart
diff --git a/pkg/analysis_server/test/services/correction/flutter_util.dart b/pkg/analysis_server/test/services/correction/flutter_util.dart
index a15dd909639fd6793fcfb55be7c3d5f1e41f9777..100f1b0d7e8588cc3b93cf5cf5914f4cddaccaf9 100644
--- a/pkg/analysis_server/test/services/correction/flutter_util.dart
+++ b/pkg/analysis_server/test/services/correction/flutter_util.dart
@@ -10,6 +10,7 @@ String get flutter_framework_code => '''
 class Widget {}
 class RenderObjectWidget extends Widget {}
 class StatelessWidget extends Widget {}
+abstract class StatefulWidget extends Widget { }
 class SingleChildRenderObjectWidget extends RenderObjectWidget {}
 class Transform extends SingleChildRenderObjectWidget {}
 class ClipRect extends SingleChildRenderObjectWidget { ClipRect.rect(){} }
@@ -19,5 +20,7 @@ class Center extends StatelessWidget { Center({child: null, key: null}){}}
 class DefaultTextStyle extends StatelessWidget { DefaultTextStyle({child: null}){}}
 class Row extends Widget { Row({List<Widget> children: null, key: null}){}}
 class GestureDetector extends SingleChildRenderObjectWidget { GestureDetector({child: null, onTap: null}){}}
-class Scaffold extends Widget { Scaffold({body: null}){}}
+class AppBar extends StatefulWidget implements PreferredSizeWidget { AppBar(title: null, color: null, key: null) }
+class Scaffold extends Widget { Scaffold({body: null, PreferredSizeWidget appBar: null}){}}
+class PreferredSizeWidget implements Widget {}
 ''';